A mortan Centurion is engaged by a Feral Hammerfist and a Feral Fleshpounder. Is it correct that the Centurion can not counter attack the Feral Fleshpounder because of the Hammerfists special rule "Threaten" ("Living enemy models engaging the Feral Hammerfist may only target the Feral Hammerfist when performing melee attacks")?

hmm this greatly increases the odds of bringing a Hammerfist. Up until now i wasn't very certain of it, but seeing as how it could disable a unit of doing counter attacks, just by beeing there, is quite good.

A followup question how ever: If a hammerfist and another unit are both in combat with the same unit(for simplicity all enemy models are in strike range of the hammerfist). For some reason my other unit moves, giving the enemy models a free attack. Am i right in thinking they can't attack the moving unit because of threaten?

I think you're correct on this one Maximus!
Counter attacks are also passive attacks like free strikes...
This makes the Hammerfist a greater pain in the a..

I yet have another question:

The hammerfist is engaged with 4 of 8 tribesmen. The other 4 are not within melee range of each other, but are within melee range of some brutes.
I suppose only the 4 that ARE in melee range are effected by threaten???
Or is the whole unit affected?